1. Field of the Invention
The present invention relates to an apparatus and method for estimating an echo path delay. More particularly, the present invention relates to an apparatus and method for estimating an echo path delay using an adaptive filter and decimators.
2. Description of the Related Art
Generally, an echo inherently exists in a long-distance telephone service between switching offices in different local exchange areas and two wire-circuits within each area connecting telephone subscribers within the switch office. As the speech energy between the two and four-wire circuits, being effected by hybrid circuits, do not perfectly match the impedance between different two-wire and four-wire circuits, the echo is an inevitable characteristic in a public switched telephone network. In order to remove the echo, an echo canceller is used to cancel the echo which would otherwise return to the far end speaker. The echo canceller estimates and cancels the echo returned from the hybrid circuits using an adaptive filter, which uses a technique known as Normalized Least Mean Squares (NLMS) adaptation. However, as the echo delay time increases, the NLMS adaptive filter experiences longer processing time due to the longer processing required computation time.
Each delay associated with the telephone service can be defined as two types: a transmission delay (TDL) and an impulse response delay (TDP). The TDL represents a normal delay time associated with the transmission of a signal. The TDP represents the impulse response duration caused by the hybrid circuits. If the units of TDL and TDP are in seconds, an echo delay time (DL) corresponds to the natural delay time (TDL) multiplied by a sampling rate, and an echo duration time (DP) corresponds to the impulse response delay (TDP) multiplied by a sampling rate. For example, if the sampling rate is 8 kHz and the measured TDL is 64 msec, the DL=8000 Hzxc3x9764 msec=512. Thus, the sum of a whole delay (D) can be represented by the following equation: D=DL+DP, and the amount of computation required by the echo canceller, per each sample, is approximately 4*D.
As explained above, the prior art echo canceller has drawbacks in that it requires much longer computation time because the whole delay time (D=DL+DP) has to be processed. For example, in the case of calculating by a million instructions per second (MIPS) at a sampling frequency of 8 KHz, the conventional echo canceller without the echo estimating device requires a computation duration of 4*D*8000 MIPS. Each sample requires four times the whole delay time for processing. Thus, the prior art canceller requires 16.384 MIPS (=4*D*8000 MIPS) of processing time if the whole each delay time is 64 msec (i.e., D =64 msecxc3x978000 Hz=512).
Since the TDP is normally less than 8 msec (or the DP is less than 64 msec), the required computation time does not exceed 4*64*8000=2.048 MIPS. Thus, if the echo delay time (DL) can be estimated, the echo delay time becomes smaller and the computation time required for canceling the echo becomes shorter and less burdensome to the system.
Accordingly, the present invention has been made in an effort to solve the problems occurring in the related art, and the object of the present invention is to provide an apparatus and method for estimating an echo path delay time.
It is another object of the present invention to provide an apparatus and method of canceling an echo by estimating only the echo factor caused by the hybrid circuits.
In order to achieve the above objects, according to the present invention, there is provided an apparatus for cancelling an echo included in a sound signal transmitted from one end speaker to a far end speaker comprising: a first low pass filter for receiving a the sound signal as a first sound signal and for filtering the first sound signal at a predetermined bandwidth; a first decimator for decimating the output signal of the first low pass filter at a predetermined decimation rate; a second low pass filter for receiving the echo caused by the first sound signal as a second sound signal and for filtering the second sound signal at a predetermined bandwidth; a second decimator for decimating the output signal of the second low pass filter at a predetermined decimation rate; an adaptive filter for receiving the output signal of the first decimator and the output signal of the second decimator to compute an echo delay estimation value; and, a canceller for subtracting the echo delay estimation value from the output signal of the second decimator.
In another aspect of the present invention, there is provided a method of estimating an echo delay in an echo delay estimating apparatus having an adaptive filter, comprising the steps of: preparing and initializing three buffers of the adaptive filter; performing a modelling with respect to an echo transfer function; searching the position of a coefficient having the largest absolute value among the coefficients of the adaptive filter; obtaining the position of the coefficient having the biggest absolute value; obtaining an actual value of the coefficient having the largest absolute value; obtaining an echo return loss enhancement (ERLE) value using an input and the output of the adaptive filter; and, obtaining an echo delay estimation value.